The present invention relates to a new compound which can be used as an amino-protecting group, a process for preparing the same, and a process for synthesizing peptides using the same. More specifically, the present invention relates to a carbonic acid ester compound of the following formula (1): 
in which
X represents para-nitrophenyloxy, imidazolyl, Cl, or Br,
Y represents S or SO2, and
Ar represents aryl group containing fluorine(s), which can be easily combined with the amino group of amino acids (capping or protection; xe2x80x98cappingxe2x80x99 below) or removed therefrom (decapping or deprotection; xe2x80x98decappingxe2x80x99 below) under mild conditions, whereby the amino group can be effectively protected during peptide synthesis, a process for preparing the same, and a use of the same. Since the amino acid derivatives thus protected have high thermal and chemical stabilities and good solubility for organic solvents, peptides can be synthesized at a low cost with a high yield.
Typically known functional groups used for protecting the amino group of amino acids when preparing peptide compounds include tert-butoxycarbonyl (tBoc), 9-fluoroenylmethoxycarbonyl (Fmoc), 2-(4-nitrophenylsulfonyl)ethoxycarbonyl (Nsc), etc.
Among them, tBoc has a fatal demerit that it cannot be used for the amino acids having functional groups sensitive to acid because it should be removed by an acid.
Fmoc can be easily decapped from the amino group under weak basic conditions, which makes the peptide synthesis possible under much milder conditions than tBoc. However, Fmoc also has some fatal demerits. First, the amino acid derivatives protected by Fmoc have low thermal and chemical stabilities, which are particularly lowered in the case of amino acids containing aromatic groups. Therefore, in order to increase the yield of peptide synthesis from the aromatic amino acid derivatives protected by Fmoc group, short reaction time and excess amount of amino acid should be applied. Second, when it is used as a protecting group of amino acids containing a heterocycle, the low solubility of the amino acids in organic solvents may not be improved to a satisfactory level. Third, differently from tBoc, methyl or ethyl ester of amino acid cannot be used as a starting material, and t-butyl ester or carboxylic acid should be used as a starting material. Particularly, since decarboxylation may easily occur in the case of heterocycle compounds, carboxylic acid cannot be used, and t-butyl ester should be used. Since this t-butyl group should be removed under strong acidic conditions, however, it is practically impossible to use the protecting group of Fmoc for the heterocycle-containing amino acids. Fourth, the base piperidine used in the decapping step of Fmoc has weak reactivity with the resulting dibenzofulvene (DBF), and so has a limit in reducing side reactions by DBF by removing side products through a reaction.
On the other hand, since Nsc is more stable than Fmoc, it is useful for peptide synthesis. However, Nsc also has the disadvantage that it is inappropriate for the synthesis on a solid phase due to the low solubility in organic solvents depending on the kind of protected compounds.
Thus, the present inventors have extensively studied to provide a process for effectively synthesizing peptides, and so have identified that the new compound of formula (1) as defined above meets the inventor""s requirements as an amino-protecting group. This new compound has the features that it has the advantage of Fmoc or Nsc, synthesis of peptides under weak basic conditions, and at the same time it cures the disadvantages thereof as illustrated above. That is, the amino acid derivatives protected by the amino-protecting group provided by the present invention have high thermal and chemical stabilities and good solubility for organic solvents; the cheap methyl or ethyl ester of amino acid can be used as a starting material in the peptide synthesis; and any side reaction by the products formed in the reaction for removing the protecting group does not occur.
Therefore, the object of the present invention is to provide the compound of formula (1) used as an amino-protecting group.
It is another object of the present invention to provide a process for preparing the compound of formula (1).
It is also another object of the present invention to provide a process for synthesizing peptides using the compound of formula (1) as a protecting group.
It is also another object of the present invention to provide an amino acid derivative protected by the compound of formula (1).
First, the present invention relates to a new carbonic acid ester compound of the following formula (1): 
in which
X represents para-nitrophenyloxy, imidazolyl, Cl, or Br,
Y represents S or SO2, and
Ar represents aryl group containing fluorine(s).
In the compound of formula (1), the substituent Ar defined as aryl group containing fluorine(s) preferably represents a radical selected from the following group: 
The carbonic acid ester compound of formula (1) according to the present invention, {circle around (1)} enables peptide synthesis under strong basic conditions, {circle around (2)} produces amino acid derivatives protected thereby, which have high thermal and chemical stabilities and good solubility for organic solvents, {circle around (3)} enables use of the cheap methyl or ethyl ester of amino acid as a starting material in the peptide synthesis, {circle around (4)} makes it easy to convert the peptides to unstable derivatives under basic conditions for easy decapping of the protecting group, and {circle around (5)} does not cause any side reaction by the products formed in the decapping reaction.
The carbonic acid ester compound of formula (1) according to the present invention can be prepared by a process characterized in that a fluorinated derivative of aryl halide of the following formula (2):
Arxe2x80x94Lxe2x80x83xe2x80x83(2)
in which Ar is defined as above and L represents a leaving group, preferably halogen, is reacted with mercaptoethanol to give a fluorinated derivative of 2-(arylsulfanyl)-ethanol of the following formula (3): 
in which Ar is defined as above; and the resulting fluorinated derivative of 2-(arylsulfanyl)-ethanol of formula (3) is reacted with a compound of the following formula (4): 
in which X is defined as above and Lxe2x80x2 represents a leaving group, preferably halogen, in order to introduce the para-nitrophenyloxy, imidazolyl, Cl, or Br group. Therefore, the present invention also relates to the above process for preparing the compound of formula (1).
Specifically, the fluorinated derivative of aryl chloride of formula (2) is dissolved in an organic solvent, mercaptoethanol is added thereto, and the mixture is reacted for 10xcx9c30 hours at 20xcx9c160xc2x0 C. to give the fluorinated derivative 2-(arylsulfanyl)-ethanol of formula (3) (Step 1). Then, the compound of formula (3) thus obtained is dissolved in an organic solvent, the compound of formula (4) is added thereto, and the mixture is reacted for 1xcx9c5 hours at 0xcx9c100xc2x0 C. to give the novel compound of formula (1) of the present invention (Step 2). For example, the reaction wherein 4-nitrophenyl chloroformate is used as the compound of formula (4) may be depicted as the following Reaction Scheme 1: 
The present invention also provides a compound of the following formula (5), 
in which Ar and Y are defined as above, and R represents an amino acid except for the amino group combined with the xcex1-carbon atom, which is an amino acid derivative of which amino group is protected by the carbonic acid ester compound of formula (1).
As stated in the following explanation on the capping step, the above compound of formula (5) is formed in the step of preparing a carbamate derivative of amino acid by reacting the compound of formula (1) according to the present invention with the amino group of an amino acid. When Y is S, the compound of formula (5) wherein Y is SO2 may be obtained by additional oxidation reaction using a suitable oxidant.
The present invention also relates to a process for synthesizing peptides using the compound of formula (1) as an amino-protecting group. That is, the present invention relates to a process for synthesizing peptides comprising
(A) the step of capping wherein the amino group of an amino acid is protected by the carbonic acid ester compound of formula (1) according to the present invention;
(B) the step of carboxylic acid formation wherein the amino acid derivative of which amino group is protected is hydrolyzed in case it is in the form of an ester;
(C) the step of peptide formation wherein carboxyl group of the amino acid derivative in the form of a carboxylic acid is coupled with amino group of another amino acid;
(D) the optional step of oxidation wherein S group at the position of Y in the capping moiety is oxidized to SO2 group in case the position of Y in the capping moiety is S group; and
(E) the step of decapping wherein the carbonic acid ester moiety is removed under basic conditions from the compound formed through the peptide formation step.
However, the oxidation step (D) may be positioned anywhere between the capping step (A) and the decapping step (E), and further the oxidation step (D) may be carried out simultaneously with the decapping step (E).
The peptide synthesizing process according to the present invention may be roughly divided into capping step, peptide formation step, and decapping step, which will be more specifically explained step by step below.
(1) Capping Step: Formation of Ttc Group
This step provides a carbamate derivative of an amino acid wherein the amino group is protected by the compound of formula (1).
In this capping step, the carbonic acid ester compound of formula (1) according to the present invention is reacted with an amino acid of formula (6) to give a carbamate derivative of an amino acid as represented by the above formula (5). This step may be more specifically depicted as Reaction Scheme 2 below: 
in which R1 and R2 each represent a substituent such as hydrogen, alkyl, aryl, or heterocycle which can be included in amino acids.
The capping group exemplified in the above Reaction Scheme 2 is 2-(4-trifluoromethylphenylthio)ethoxycarbonyl (xe2x80x98Ttcxe2x80x99 below) which is stable against a base.
(2) Carboxylic Acid Formation Step
When the right side terminal of the above compound of formula (5), a carbamate derivative of capped amino acid, is not carboxylic acid, that is, when R2 group is not hydrogen, the terminal is converted to carboxylic acid using a base. This step may be depicted, for example, as the following Reaction Scheme 3. In the following Reaction Scheme 3, the capping group is Ttc. 
(3) Peptide Formation Step
Then, the carboxylic acid at the right terminal of the compound of formula (5) is reacted with an amino group of another amino acid to form a peptide bond and to give an amino acid dimer derivative such as the compound of the following formula (7). 
in which
Ar and R1 are each defined as above, and
R1xe2x80x2 represents a substituent such as hydrogen, alkyl, aryl, or heterocycle which can be included in amino acids.
As the agent that can be used in the peptide bond formation of the above compound of formula (7), tetramethylfluoroformamidinium hexafluorophosphate(TFFH), bromo-tris-pyrrolidino-phosphonium-hexafluorophosphate(PyBroP), etc. may be mentioned, but not restricted thereto.
The above explained two steps (carboxylic acid formation step+peptide formation step) are repeated until the desired amino acid polymer (peptide) is obtained and finally the right terminal of the peptide derivative is converted to carboxylic acid.
(4) Oxidation Step: Formation of Tsc Group
At any time after the capping step, in case Y is S in the compound of formula (5), it can be oxidized to SO2 in order to increase the sensitivity of the cap (carbonic acid ester compound) to the base. That is, as depicted in the following Reaction Scheme 4, the S moiety of the Ttc group is oxidized to SO2 moiety. 
Specifically, the starting compound containing a thio group (xe2x80x94Sxe2x80x94) is treated by hydrogen peroxide or a mixture of hydrogen peroxide and other oxidant in an organic solvent at a temperature ranging from xe2x88x9210xcx9c100xc2x0 C. to give an oxidized product containing a sulfonyl group (xe2x80x94SO2xe2x80x94). The oxidized product as exemplified in the above Reaction Scheme 4 is a compound containing 2-(4-trifluoromethyl-benzenesulfonyl)ethoxycarbonyl (xe2x80x98Tscxe2x80x99 below) group, of which carbamate bond becomes very weak against a base.
The oxidation step can be performed at any time after the capping step, and if necessary, carried out simultaneously with the following decapping step.
(5) Decapping Step
To the compound obtained from the above explained oxidation step is added a secondary base such as piperidine, or a tertiary or aromatic base in an organic solvent in order to decompose the carbamate bond and to give a peptide of the following formula (8) in the decapping step: 
in which R1 and R1xe2x80x2 are each defined as above.
That is, in this step, the peptide compound synthesized through the peptide formation step is hydrolyzed under basic conditions as the case of Nsc, Fmoc, etc. to remove the carbonic acid ester moiety. For example, the Tsc group can be removed via xcex2-hydrogen elimination reaction with 20%(v/v) piperidine in DMF solvent.
For reference, after the protecting group Tsc is decapped by piperidine, the resulting products are analyzed by NMR. As a result, as depicted in the following Reaction Scheme 5, only the compound of formula (10) is detected as the decapping product, and the compound of formula (9), a side product of the xcex2-hydrogen elimination reaction, is not. 
Therefore, since such compounds that may cause side reactions in the decapping step as the compound of formula (9) are not produced in the process according to the present invention, the desired peptide can be more effectively obtained than in the previous process using the Fmoc group.
The present invention will be more specifically explained in the following examples. However, it should be understood that the following examples are intended to illustrate the present invention but not to limit the scope of the present invention in any manner. Further, a person skilled in the art may easily modify the reactants and reaction conditions in the following examples within a reasonable range, and such modifications also fall within the technical scope of the present invention.
Usually, there is a problem of solubility decrease of the reactants (intermediates) to the solvent in the artificial synthesis of peptides. Therefore, in order to confirm that there is no problem caused by the solubility decrease of the reactants (intermediates) in the process of the present invention, amino acids having such N-containing heterocycles as pyrrole or imidazole, that is, those having low solubility, are particularly selected and used as the reactants in the following examples.
The full names of the abbreviations used in the following examples are as follows:
EA: ethyl acetate
DMF: dimethylformamide
DCM: dichloromethane
DIEA: diisopropylethylamine
HOBt: 1-hydroxybenzotriazole
THF: tetrahydrofuran
TFFH: tetramethylfluoroformamidinium hexafluorophosphate
DMAP: 4-(N,N-dimethylamino)pyridine
PyBroP: bromo-tris-pyrrolidino-phosphonium-hexafluorophosphate